$f$-mode strengthening from a localized bipolar subsurface magnetic field

TL;DR

This study models localized bipolar subsurface magnetic fields and their impact on the $f$-mode in helioseismology, revealing how magnetic field strength and depth influence $f$-mode behavior and its potential as a solar magnetic field tracer.

Contribution

It introduces a more realistic localized magnetic field model and analyzes its effects on the $f$-mode, contrasting with previous periodic magnetic field studies.

Findings

Localized magnetic fields increase $f$-mode strength at high wavenumbers.

Fanning of the $f$-mode is weaker with localized fields than with periodic ones.

$f$-mode perturbations could serve as early indicators of subsurface solar magnetic activity.

Abstract

Recent numerical work in helioseismology has shown that a periodically varying subsurface magnetic field leads to a fanning of the $f$-mode, which emerges from the density jump at the surface. In an attempt to model a more realistic situation, we now modulate this periodic variation with an envelope, giving thus more emphasis on localized bipolar magnetic structures in the middle of the domain. Some notable findings are: (i) compared to the purely hydrodynamic case, the strength of the $f$-mode is significantly larger at high horizontal wavenumbers $k$, but the fanning is weaker for the localized subsurface magnetic field concentrations investigated here than the periodic ones studied earlier; (ii) when the strength of the magnetic field is enhanced at a fixed depth below the surface, the fanning of the $f$-mode in the $k\omega$ diagram increases proportionally in such a way that the normalized $f$-mode strengths remain nearly the same in different such cases; (iii) the unstable Bloch modes reported previously in case of harmonically varying magnetic fields are now completely absent when more realistic localized magnetic field concentrations are imposed beneath the surface, thus suggesting that the Bloch modes are unlikely to be supported during most phases of the solar cycle; (iv) the $f$-mode strength appears to depend also on the depth of magnetic field concentrations such that it shows a relative decrement when the maximum of the magnetic field is moved to a deeper layer. We argue that detections of $f$-mode perturbations such as those being explored here could be effective tracers of solar magnetic fields below the photosphere before these are directly detectable as visible manifestations in terms of active regions or sunspots.